How to make texture not repeat
There were multiple problems with the mesh, but it all came down to materials being assigned to the wrong places. The shirt material was assigned to the head, mouth, and eyes. Create new material slots for these areas, select the already working-on-other-character materials from the list of options for each slot rather than creating new ones, and assign them to the right places. Since several duplicate vertices exist, I personally recommend selecting all and merging by distance first, then using Select Linked but limited to UV to easily select one-material areas.
For more information, see the Blender Manual on Material Assignment